1. Field of Invention
The present invention relates to a method of injection molding a low-melting-point alloy, and more particularly to a method of injection molding a low-melting-point alloy in which a low-melting-point alloy, which is a molding material, is melted in a cylinder of a screw-type low-melting-point alloy injection molding machine and is injected into a mold by a screw to effect molding, and control is provided such that an appropriate quantity of material is fed from a material feeder in correspondence with a shot and the quantity of material detained in the cylinder is maintained appropriately.
2. Related Art
Conventionally, methods of injection molding a low-melting-point alloy include a pressure die casting process and a metal injection molding process. The low melting point referred to herein means a temperature up to 700xc2x0 C. or thereabouts, and as specific materials which can be used, Al, Mg, Zn, Bi, Sn, and Pb alloys correspond to them. As shown in FIG. 1a, a low-melting-point alloy injection molding machine using an in-line type screw as in the metal injection molding process is comprised of a screw-type, belt-type, or a vibration-type material feeder 16 for feeding material chips of a low-melting-point alloy, a screw 11, a cylinder 12, and a heater 13 for transporting and melting the fed material, a nozzle 14 and a high-speed injecting apparatus 17 for injecting and ejecting the molten material into a mold 19, and so on.
With the above-described low-melting-point alloy injection molding machine, a metal in chip form which is obtained by cutting ingots, as shown in FIG. 1b, is used as a raw material. In the low-melting-point alloy injection molding machine, the material stored in the hopper of the material feeder 16 is fed into the cylinder 12 by the material feeder 16. The material thus fed is transferred forwardly inside the cylinder 12 by the rotation of the screw 11 disposed in the cylinder 12 so as to be rotatable and reciprocatable. The material, while being transferred, is set in a predetermined molten state on heating by the heater 13 (an electric heater, a induction heater, or the like) attached to an outer peripheral portion of the cylinder 12.
At the time of the above-described heating, the screw 11, while rotating in the cylinder 12, retains the material in the molten state (a molten light metal) in a retaining section 15 at a distal end of the screw 11, and retracts to the material feeder side at a fixed speed. The material in the molten state retained in the retaining section 15 is metered by the amount of this retraction. Namely, at the point of time when the screw 11 reaches a predetermined retraction stroke as the screw 11 rotates, the rotating and reciprocating motion of the screw 11 are finished, thereby completing the metering of a predetermined quantity of a molten light metal corresponding to the retraction stroke. In the injection process after completion of the metering, the screw 11 is advanced at high speed by the high-speed injecting apparatus 17, and the molten light metal metered in the retaining section 15 is injected into the mold 19 through the nozzle 14 at the distal end of the cylinder 12.
With such a low-melting-point alloy injection molding machine, there are cases where, during the metering operation, the rotational load pressure of the screw rises, and the number of revolution of the screw varies. If such a situation occurs, variations occur in the quantity of molten metal injected in an ensuing shot. It is experientially known that the rise of the rotational load pressure is related to the quantity of material detained in the cylinder and occurs when the quantity of material detained exceeds an upper limit. That is, the quantity of material detained is determined by the relationship between, on the one hand, the quantity of material chips (quantity of material chips=Qin) fed from the material feeder and, on the other hand, the quantity of molten metal discharged (shot weight=Qout) discharged into the mold in the injection process for each shot. The following situation occurs depending on the relationship between their relative magnitudes:
(A) In the case of Qin greater than Qout:
Since the quantity of material fed is large relative to the shot weight, the quantity of material detained, Q, inside the cylinder increases by
xcex94Q(=Qinxe2x88x92Qout)
for each shot. Consequently, increments xcex94Q of the detained quantity are accumulated with the advance of molding, and there are cases where an abnormality occurs to the rotation of the screw when the quantity of material detained, Q, has exceeded an upper limit Qmax.
(B) In the case of Qin less than Qout:
Since the quantity of material fed is small relative to the quantity of molten metal discharged, the quantity of material detained, Q, decreases by
xcex94Q(=Qoutxe2x88x92Qin)
for each shot. Consequently, decrements xcex94Q of the detained quantity are accumulated with the advance of molding, and a short shot occurs when the quantity of material detained, Q, has fallen below a lower limit Qmin.
As described above, the quantity of material detained in the cylinder exerts a large influence on the stability of the metering of the low-melting-point alloy injection molding machine, so that, in practice, a balance is established between them, and adjustment of molding conditions is carried out so as to effect molding in a state in which a fixed quantity of material is detained. As a specific countermeasure, while observing the change in the rotational load pressure or the state of filling of molded articles, and the like, settings of the quantity of material fed, q, per unit time or the feeding time ts of the material feeder, and the like are adjusted on the basis of experience so as to feed the material in a quantity commensurate with the shot weight. However, it has been difficult to stably and continuously maintain the quantity of material detained due to the variation of the shot weight and the variation of the quantity of material fed by the material feeder, or due to a change in the molten state of the material in the cylinder in consequence of the temporary stopping of the machine.
The invention has been devised to overcome the above-described problems, and its object is to provide a method of injection molding a low-melting-point alloy in which an upper limit and a lower limit of the quantity of material detained in the cylinder are set to eliminate an excess or a shortage of the quantity of material detained in the cylinder in the process of injection molding a low-melting-point alloy, and which is capable of automatically maintaining the quantity of material detained in the cylinder within the range between the set upper and lower limits.
To overcome the above-described problems, in accordance with the invention there is provided a control method of an injection molding in which a low-melting-point alloy, which is a molding material, melted in a cylinder of a screw-type low-melting-point alloy injection molding machine and injected into a mold by a screw to effect molding, and control is provided such that an appropriate quantity of material is fed from a material feeder in correspondence with a shot and the quantity of material detained in the cylinder is maintained appropriately, comprising the steps of:
setting a unit increment (xcex94Q+) and a unit decrement (xcex94Qxe2x88x92) as well as an upper limit (Qmax) and a control lower limit (CQmin) of a quantity of material detained; and
automatically repeating the operation in which an injection process is executed by feeding the material in a quantity
(Qin=Qout+xcex94Q+)
xe2x80x83which is the unit increment (xcex94Q+) greater than each shot weight (Qout), and when the quantity of material detained (Q) in the cylinder has reached the upper limit (Qmax) due to the accumulation of the unit increments (xcex94Q+) the quantity of material fed (Qin) is changed to a quantity
(Qin=Qoutxe2x88x92xcex94Qxe2x88x92)
xe2x80x83which is the unit decrement (xcex94Qxe2x88x92) less than the shot weight (Qout), and in which
after the change, the injection process is further executed, and when the quantity of material detained (Q) in the cylinder has reached the control lower limit (CQmin) due to the accumulation of the unit decrements (xcex94Qxe2x88x92), the quantity of material fed (Qin) is returned to a quantity which is the unit increment (xcex94Q+) greater than the shot weight (Qout) so as to increase the quantity of material detained (Q), thereby optimally maintaining the quantity of material detained (Q) in the cylinder.
In accordance with the above-described arrangement, when the quantity of material detained (Q) is between the upper limit (Qmax) and the control lower limit (CQmin), and when the material is fed in a quantity
(Qin=Qout+xcex94Q+)
which is the unit increment (xcex94Q+) greater than the shot weight for each shot, the quantity of material detained (Q) in the cylinder reaches the upper limit (Qmax) due to the accumulation of the unit increments (xcex94Q+). Accordingly, the quantity of material fed (Qin) is changed to a quantity
(Qin=Qoutxe2x88x92xcex94Qxe2x88x92)
which is the unit decrement (xcex94Qxe2x88x92) less than the shot weight (Qout). Accordingly, when the injection process is repeated, the quantity of material detained (Q) in the cylinder reaches the control lower limit (CQmin) due to the accumulation of the unit decrements (xcex94Qxe2x88x92). Accordingly, the quantity of material fed (Qin) is returned to a quantity which is the unit increment (xcex94Q+) greater than the shot weight (Qout) so as to increase the quantity of material detained (Q). As these changes of the quantity of material fed (Qin) are automatically repeated, the quantity of material detained (Q) in the cylinder is maintained within the range between the upper limit (Qmax) and the control lower limit (CQmin), thereby making it possible to prevent an abnormality of the rotation of the screw and short shots. It should be noted that, in the invention, the shot weight (Qout) is measured automatically by a weight measurement sensor or manually to calculate that the quantity of material detained (Q) has reached the upper limit (Qmax) and the control lower limit (CQmin).
In addition, in the invention, at least one of the rotational load pressure, back pressure, and screw rotating time during the rotation of the screw is used to detect the reaching of the upper limit (Qmax). As a result, the invention can be implemented by using a detecting means which is conventionally disposed.
In addition, in the invention, in the case where the quantity of material fed is changed, the unit increment (xcex94Q+) and the unit decrement (xcex94Qxe2x88x92) are changed in steps. This is to prevent the molten state of the material in the cylinder from changing suddenly and to prevent the occurrence of an abnormality in the quality.
In addition, in the invention, in a case where changes are made in steps, the unit increment (xcex94Q+) and the unit decrement (xcex94Qxe2x88x92) themselves which are applied in a prescribed time duration are changed, or the applicable prescribed time duration is changed. This facilitates the programming of a control program.
In addition, in the invention, alloy chips of one of Mg, Al, Zn, Sn, Pb, and Bi are used as the molding material. These materials are easily available, and are suitable to the implementation of the invention in the light of their properties.
To sum up the major points of the foregoing description and give a further description, the control method of the injection molding for stabilizing metering without causing a rise in the rotational load pressure is to control the quantity material detained within upper and lower limits by directly or indirectly estimating the quantity of material detained in the cylinder. Namely, the material is fed in a quantity
(Qin=Qout+xcex94Q+)
which is xcex94Q+ greater than the shot weight Qout, and when the quantity of material detained in the cylinder has reached the upper limit Qmax due to the accumulation of xcex94Q+, the quantity of material fed Qin is changed to a quantity
(=Qoutxe2x88x92xcex94Qxe2x88x92)
which is xcex94Qxe2x88x92 less than the shot weight Qout, so as to decrease the quantity of material detained in the cylinder. On the other hand, in a case where the quantity of material detained is on a decreasing trend, at the point of time when the quantity of material detained has reached the control lower limit CQmin which is set at a level lower than the lower limit Qmin of the quantity of material detained which was determined experimentally, the quantity of material fed Qin is changed again to a value which is xcex94Q+ greater than the shot weight Qout so as to increase the quantity of material detained. Thus, stable metering is realized by effecting molding while repeating the decrease and increase of the quantity of material detained in the cylinder within the range between the upper and lower limits.